1. Field of the Invention
The present invention relates to a display control device for the so-called partial rewrite of the display contents on a liquid crystal display unit, and more particularly to a display control device suitable in combination with a liquid crystal display unit using a ferroelectric liquid crystal having memory property.
2. Related Background Art
Conventionally, refresh scan-type CRTs have been mainly used as the computer terminal display, and vector scan-type CRTs having memory property have been partially used as the CAD oriented large-scale high definition display. A vector scan-type CRT is unsuited for the man-machine interface display for use in real-time, such as the cursor shift display, the icon-based display useful for the information display from a pointing device such as a mouse, and the edit display (insert, delete, move, copy) of characters and texts, because the picture, once displayed, is not updated until it is erased.
On the other hand, a refresh scan-type CRT requires a refresh cycle of 60 Hz or greater as the frame frequency from the viewpoint of preventing the flicker (picture flicker), and adopts a non-interlace method to improve the visibility in the shift display (shift display of icon) of the information within a screen (note that TV adopts a 1/2 interlace method with a field frequency of 60 Hz and a frame frequency of 30 Hz from the consideration of the display of moving picture and the simplicity of drive control system). Therefore, with a higher display resolution, the display unit is larger, resulting in a higher power, a larger drive control and a higher cost.
In the background of the recent advent of flat panel displays, there is the inconvenience with such larger and higher power CRTs.
At present, several types of flat display panels have been known. For example, a high time-division drive system (STN) with the twist nematic liquid crystal, its variation for the white and black display (NTN), or a plasma display system, takes the same image data transfer method as that for the CRT, with its picture update method being a non-interlace method having a frame frequency of 60 Hz or greater. A large-size flat display panel in which the total number of scanning lines in one picture is from 400 to 800 lines, and 1000 lines or greater, has no memory property on the driving principle, and thus requires a refresh memory having a frame frequency of 60 Hz or greater to prevent the flicker. Accordingly, one horizontal scanning time was as short as 10 to 50 .mu.sec or less, and excellent contrast could not be attained.
A ferroelectric liquid crystal display has a memory property, and is able to make a display on a larger screen and at a higher definition than with the above-mentioned display. However, by virtue of driving at its low frame frequency, to cope with the display unit with the man-machine interface as previously described, a partial rewrite scanning (for scanning only the scanning lines within a rewrite region) which can make effective use of the memory property has been recommended. This partial rewrite scanning system has been disclosed in, for example, U.S. Pat. No. 4,655,561 by Kanbe.
This partial rewrite scanning system is based on a method in which the partial rewrite scanning is performed by designating a partial rewrite scanning start address and end address, and a method of using a circuit (e.g., a timer) for controlling the partial rewrite scanning time.
Among them, the method of using a circuit for controlling the partial rewrite scanning time allows for other image processing instructions or partial rewrite scanning during the partial rewrite scanning, whereby the display with the mouse or cursor shift can be made during the scroll display on a multi-window. However, with the conventional method, in making other partial rewrites during a partial rewrite scanning, the partial rewrite scanning region was designated for each partial rewrite request so that if the partial rewrite scanning region overlapped, duplicate scanning was performed in the same scanning region. Thereby there was a problem that the partial rewrite process might take a more time than necessary.
For example, in operation with the window scroll display and the pointing device display, it is assumed that a partial rewrite scanning request for the window scroll display is first generated, and then a display request from a pointing device is generated after the partial rewrite scanning with scroll on the display panel. The rewrite display for the pointing device will be immediately conducted, and then the scroll display will be made again, but the method of designating the partial rewrite scanning region with the partial rewrite request itself for the scroll display had a problem that if the pointing device existed within the scroll area, a region already displayed by the partial rewrite of the pointing device was scanning again by the scroll partial rewrite, so that duplicate scanning was made, taking more time than necessary to complete a partial rewrite process.
Also, with the conventional method, when a partial rewrite instruction having the same level of priority was generated during the partial rewrite process, either a method of storing no image information or a method of storing only the image information and not performing the partial rewrite until the termination of the partial rewrite being currently executed was adopted. This is due to the fact that as the partial rewrite scanning region is designated for each partial rewrite request, when one partial rewrite is being currently executed, other partial rewrites must be either delayed until the end or ignored. Accordingly, there was a problem that in a former method, the partial rewrite process required a greater time, and in a latter method, the display was not enabled.